Electrocoating process

ABSTRACT

IN A PROCESS OF COATING AN ELECTRICALLY CONDUCTIVE ARTICLE BY ELECTRODEPOSITION FROM A DISPERSION OF FILM-FORMING MATERIAL, THE COATING OF A SURFACE OF THE ARTICLE RELATIVELY INACCESSIBLE TO THE COUNTER-ELECTRODE IS IMPROVED BY JOINING THE ADJACENT REGION OF DISPERSION THROUH AN ION-CONDUCTING BRIDGE WITH A MORE ACCESIBLE REGION. THE BRIDGE MAY COMPRISE AN ELECTROLYTE WITHIN A CASING WHICH IS PERMEABLE TO THE COUNTER-ION.

United States Patent US. Cl. 204-181 12 Claims ABSTRACT OF THE DISCLOSURE In a process of coating an electrically conductive article by electrodeposition from a dispersion of film-forming material, the coating of a surface of the article relatively inaccessible to the counter-electrode is improved by joining the adjacent region of dispersion through an ion-conducting bridge with a more accessible region. The bridge may comprise an electrolyte within a casing which is permeable to the counter-ion.

BACKGROUND OF THE INVENTION (1) Field This invention relates to a process of coating an electrically conductive article by electrodeposition and to apparatus for use in the process.

(2) Prior art It is well established to coat an electrically conductive article by electrodeposition from a dispersion of charged particles or micelles of film-forming coating material, the article being immersed in the dispersion and an electric current passed between the article and a counter electrode which is in electrical contact with the dispersion. The polarities of the article and counter electrode are such that discharged film-forming coating material is deposited on the article and counter-ions are discharged at the counter electrode. A wide range of articles is suitable for coating by this process and include, for example, automobile bodies and other elaborate structures, the process being particularly advantageous where good coverage of a substrate is required by a coating which is intended to prevent corrosion of the substrate. Despite clear advantages in certain respects over prior processes, however, such as the ability to coat sharp edges, there may be unsatisfactory coating of surfaces located in hollows, cavities, recesses, box sections, and in general those surfaces of an article which are adjacent regions of the dispersion which are less electrically accessible to the counter electrode than other regions of the dispersion.

By the electrical accessibility of a given region of the dispersion with reference to a counter electrode, we mean a characteristic which is the inverse of the electrical resistance between that region and the counter electrode. By way of illustration, when a hollow article having a narrow entrance to its interior, e.g. a petrol tank, is immersed in a dispersion of film-forming material and is coated by passing electric current between the article and a counter electrode, a region of the dispersion which is inside the article has a lower electrical accessibility than a region of the dispersion which is outside the article.

In order to improve the coating of surfaces located in hollows, box sections, and the like, and by analogy with established practice in the electroplating art, it has been proposed to locate one or more auxiliary counter electrodes in those regions of the dispersion adjacent the article which are of reduced electrical accessibility. These auxiliary counter electrodes require an independent supply of electricity, however, and manpower is expended 3,699,031 Patented Oct. 17, 1972 in connecting and electrically testing them for short circuits before the coating process. Moreover, since the electrodeposition process involves the movement and eventual discharge of counter-ions under the influence of an electric potential gradient, there is an accumulation of the products of the discharge of counter-ions in the vicinity of the auxiliary counter electrode and in the coating bath generally. As a result the coating process becomes increasingly inefiicient unless steps are taken to prevent this accumulation.

SUMMARY We have found that the coating by electrodeposition of surfaces adjacent the less electrically accessible regions of a given dispersion of film-forming material can be improved without the use of auxiliary counter electrodes.

According to this invention we provide a process of coating an electrically conductive article wherein the article is immersed in a dispersion of ionised film-forming material and counter-ion and an electric current is passed between the article and a counter electrode in electrical contact with the dispersion, a first region of the dispersion being joined by an ion-conducting bridge as herein defined to a second region of the dispersion which has a lower electrical accessibility to the counter electrode than has the first region.

By ion-conducting bridge we mean a bridge between a first region of the dispersion and a second region which is impermeable to the ionised film-forming material but permeable to ions of the opposite sign, and which, under the influence of an electric potential gradient, permits the transfer of ionic charges of the same sign from the first region to the second region more readily than those ionic charges would be conducted in the absence of the bridge, e.g. by dispersion which has been displaced by the bridge. The coating of surfaces of an article which are adjacent relatively electrically inaccessible regions of a dispersion can thus be improved by connecting these regions through the bridge to regions of higher electrical accessibility by displacement of dispersion. Alternatively it may be required to provide a route for ions which is at least partially external of the dispersion in which the article is immersed, the ions being more readily conducted along this route than they could be, if at all, by the dispersion itself.

DETAILED DESCRIPTION The ion-conducting bridge may withdraw counter-ions selectively from the one region and present ions of the same sign selectively to the other region, and in a preferred form of the invention the ion-conducting bridge is comprised of an ion exchanger. Ion exchangers are well known in various physical forms such as beads, sheets or membranes and are selectively permeable to ions of one sign depending upon their chemical structure. They are also capable of electrolytically conducting those ions of given sign to which they are selectively permeable. The ion-conducting bridge for use in the present invention may, therefore, consist essentially of an ion exchanger, for example the bridge can comprise a rod, bar, tube or other elongated structure fabricated from an ion exchanger.

Suitable ion exchangers of which the ion-conducting bridge for use in the present process may be comprised include, for example, sulphonated crosslinked polystyrene, e.g. sulphonated styrene/divinylbenzene copolymers or chloromethylated and aminated crosslinked polysty- The conductivity through a bridge, consisting essentially of an ion exchanger, of ions of the same sign as those to which the ion exchanger is selectively permeable, is determined entirely by the nature of the ion exchanger employed to achieve selectivity and this is selected to suit the circumstances.

Many ion exchangers of suitable conductivity have poor mechanical strength and as one feature of this invention the bridge comprises an ion exchanger protected by a casing of higher mechanical strength. Examples of such a bridge include an ion exchanger protected by (i) ion exchange membrane of higher mechanical strength (ii) dialysis membrane, e.g. fabric, regenerated cellulose (iii) mesh or porous or perforated sheet or plastics material (iv) impervious plastics material and having at each end of the elongated bridge an area of material such as is described in (i), (ii) or (iii) above.

In another embodiment, the ion conductivity through the bridge is improved by using an electrolyte or polyelectrolyte which has better conductivity than that of the dispersion displaced, and according to a further feature of the invention we provide an ion-conducting bridge for use in electrodeposition which comprises an electrolyte or-polyelectrolyte enclosed in an elongated casing having at opposite ends areas which are permeable to ions of sign opposite to that of the film-forming material. Preferably the casing is comprised of an ion exchanger. Improved conductivity may be achieved, for example, as a result of a higher ion concentration in the bridge than in the dispersion and/or by higher ionic mobility in the bridge. If, e.g., the counter-ion of the dispersion is an alkali metal ion, the conductivity of ionic charges through the bridge is improved, e.g. by the presence of a higher concentration of alkali metal ions in the bridge and/or by the presence in the bridge of an ion of the same sign as but higher mobility than the ionised film-former.

The conduction of ions between the ion exchanger or other interfaces with regions of dilfering electrical accessibility may be affected in media which have the required improved electrolytic conductivity, for example, solutions or gels of simple electrolytes; solutions, dispersion, or gels of polyelectrolytes or swollen solid particles or polyelectrolyte in contact with each other. The ions conferring conductivity upon such solutions, dispersions, gels or particles towards the region of lower electrical accessibility would normally be chosen to be the same as those which are to be withdrawn, preferably selectively, from the one region and presented to the other region of the dispersion of film-forming material. Suitable electrolytes include, for example, the hydroxides or salts of the alkali metals, particularly potassium, and organic salts such as potassium phenol sulphonate.

In certain ciricumstances, however, the conducting ions may be different from the counter-ion of the dispersion e.g. if the ions to be withdrawn from the dispersion are incapable of providing sutlicient electrolytic conductivity within the bridge. In this case the bridge will present to the dispersion ions of a different nature from those withdrawn from the dispersion and so the bridge will have a working life limited by its initial content of conducting ions. For example, the counter-ions in the dispersion may be derived from an amine and higher conductivity may be imparted in the bridge by using, say, alkali metal ions, particularly potassium ions. If it is desired to employ an amine salt as electrolyte within the bridge suitable highly dissociated salts include the sulphonate salts of amines.

In another form the casing of the ion-conducting bridge may comprise conventional dialysis membrane which is in contact with the regions of the dispersion of different electrical accessibility, the membrane being permeable to counter-ion and substantially impermeable to ionised film-forming material. Typical such membranes include for example those fabricated from cellulose or polypropylene. In such a bridge the counter-ion or an ion of like sign may be conducted through the bridge by any of the ion-conducting media referred to above, for example the conducting medium may comprise a higher concentration of the same ionised film-forming coating material which is present in the coating bath, or an ionised material of which the counter-ion can permeate the dialysis membrane but the other ion is unable to permeate the membrane e.g. linseed oil fatty acid at least partially neutralised with an amine or inorganic base.

By a dispersion of film-forming material we mean that a film-forming material is dispersed in a continuous medium in such form as will permit it to be electrodeposited onto an article. Thus, for example, the film-forming material may be present in solution, as indicated by the visual absence of particles; or as a colloidal suspension in which particles may or may not be visible; or as an emulsion in which liquid particles are suspended in a continuous medium, or as a suspension of visible solid particles in a continuous medium. The medium may contain suitable additives to assist the dispersion of the filmforming material or the operation of the electrodeposition process. There may also be present other coating constituents, for example, pigments, fillers and plasticizers.

The present process is applicable to dispersions of a wide range of film-forming materials, for example, the acrylic polymers and copolymers, alkyd resins and epoxy resins and these materials may be dispersed in an aqueous or non-aqueous medium. However, the process is particularly applicable to dispersions of film-forming materials in an aqueous medium. Acidic materials which are suitable for dispersion in an aqueous medium include polycarboxylic acid resins such as alkyd resins, addition copolymers containing free carboxylic groups, maleinised oils, maleinised fatty acid esters of polyols, and esters formed by polyols (including epoxy resins) with fatty acids and maleinised fatty acids. These materials may be dispersed in aqueous medium when at least partially neutralised with a base, for example, ammonia, a water soluble amine or an inorganic base such as sodium or potas-- sium hydroxide, and when employed in the electrodeposition process according to this invention the ion-conducting bridge must be permeable to the base counter-ion. Conversely, when the film-forming material dispersed in aqueous medium is a basic material such as an acrylic polymer or a polyamide containing basic groups, e.g. amino groups, it may be at least partially neutralised with an acid, for example, phosphoric acid and the ion-conducting bridge must be permeable to the acid counter-ion.

In carrying out the process of this invention the ionconducting bridge may be located in any suitable position in relation to the article so that the regions of dilfering electrical accessibilities are joined. For example, the bridge may be temporarily attached to the article or loosely located in a recess whilst coating takes place.

It'may be found that the end of the bridge adjacent the region of lower electrical accessibility becomes coated with a layer of film-forming material. This may be removed by reversing the directions of the bridge when it is again used in the process.

Articles may be coated singly or continuously whilst immersed in the dispersion of film-forming material and it is a particular advantage that the bridge is employed in relation to an article being coated without electrical wiring and without any requirement for testing for the absence of a short circuit with the article.

It is also an advantage of the present process that the coating of surfaces of an article relatively inaccessible to the counter electrode may be effected whilst maintaining good control of the counter-ion content, and thus the pH, of a coating bath by the process described in our British Pat. No. 1,106,979.

The invention is illustrated in the following examples.

Example 1 A cylindrical container was constructed from a cation exchange membrane commercially available as Ionac MC 3470 (Ionac is a Registered Trademark) filled with by weight aqueous potassium hydroxide solution and then completely sealed. Liquid-tight bonding of the membrane forming the walls of the sealed cylinder was achieved with a neoprene adhesive applied from solution in a blend of ketone and aromatic solvents. The filled, sealed cylinder was a selective ion-conducting bridge according to the invention.

One end of the filled cylinder was inserted through the single narrow opening in a hollow vessel fabricated in sheet metal, so that approximately half of the length of the cylinder was within the vessel, and the whole immersed in an aqueous dispersion of an epoxy ester polycarboxylic acid resin of acid value 85 mg. KOH/g. in which the acid groups were partially (60%) neutralised with 0.1 N potassium hydroxide. The other end of the cylinder was located in the main body of the dispersion. Suflicient space was allowed between the bridge and the opening in the vessel to permit dispersion to fill the vessel. An electric current was passed at 200 volts between the metal vessel and a counter electrode in electrical contact with the dispersion for suficient time to deposit a coating of desired thickness on the exterior of the vessel. When coating was complete, the cylinder was removed and it was found that the hollow interior of the vessel had 'received a coating of substantially the same thickness as the exterior. In a similar experiment but in the absence of the cylinder the coating received by the interior of the vessel was substantially less than that received by the exterior of the vessel.

Example 2 A cylindrical container was constructed from polyvinyl chloride and a portion of the wall near to each end of the cylinder was replaced by cation exchange membrane commercially available as Ionac 3470. A liquid-tight bond between the polyvinyl chloride and the ion exchange membran was obtained by using a neoprene adhesive. The cylinder was filled with 5% by weight aqueous potassium hydroxide and then completely sealed.

One end of the cylinder was inserted through the single narrow opening of a hollow metal vessel so that the portion of the cylinder wall replaced by cation exchange membrane near to that end was within the vessel, and the whole was immersed in an aqueous dispersion as described in Example 1. As in Example 1 when electric current was passed between the metal vessel and a counter electrode the thickness of coating on the interior of the vessel was substantially the same as the coating on the exterior. Similarly when the cylinder was absent the coating received by the interior of the vessel was substantially less than that received by the exterior.

Example 3 In this example there is illustrate the coating of the interior and exterior of an automobile petrol tank.

A cylindrical container of length 12 inches and diameter 2 inches was constructed around an axial PVC rod which had two terminal circular PVC discs each adapted to receive cylinder walls of cation exchange membrane (commercially available as Ionac 3470) so that there was a liquid-tight seal between the discs and the membrane. The cylinder walls consisted of two separated cylindrical lengths of cation exchange material, the outer ends of each length being sealed to the terminal discs and the inner ends joined by and sealed to a collar supported on stems situated mid-way along the length of the rod. The collar served to strengthen the walls and the membrane Was further reinforced by a rigid plastic mesh material which was arranged to cover the surface of the membrane within the cylinder. The collar also carried projections suitable for locating the container with respect to the entrance to the petrol tank. The container was filled with a l N aqueous solution of potassium phenol sulphonate through a port in one of the terminal discs which was then sealed to provide a completely liquid-tight and filled container.

The cylindrical container was inserted through the entrance hole to a bafiled petrol tank having a total internal area of 10 square feet and entrance hole diameter of approximately 3 inches. Projections from the collar were used to locate the container with respect to the entrance hole so that approximately half of its length was within the tank and the walls were out of contact with the tank. The tank and the positioned cylindrical container were then completely immersed in an aqueous dispersion of an epoxy ester polycarboxylic resin of acid value mg. KOH/g. partially neutralised (60%) with potassium hydroxide (0.1 N).

An electric current was passed between the tank as anode and another electrode in electrical contact with the dispersion for 4 minutes at 200 volts, the current falling from 32 amps to 16 amps over this time. The exterior of the petrol tank had a coating of approximately 0.001 inch and the interior a coating of approximately 0.0007 inch. When the petrol tank was coated under the same conditions except that the cylindrical container was absent, the exterior coating thickness was approximately 0.001 inch and the interior thickness approximately 0.0002 inch in the region of the tank entrance and negligible elsewhere.

We claim:

1. In a process of coating an electrically conductive article by electro-deposition from an aqueous dispersion of an ionized film-forming material and a counter-ion, said counter-ion serving to disperse said ionized filmforming material in said dispersion, said process comprising immersing said article in said dispersion as one electrode spaced from a counter-electrode in electrical contact with said dispersion and establishing an electric potential gradient between said article and said counter electrode to cause deposition of a coating of said filmforming material on said article, and said article having a surface adjacent a second region of the dispersion which region is less electrically accessible to the counterelectrode than is a first region of said dispersion which communicates with said second region and surrounds said article;

the improvement which comprises increasing the electrical accessibility of the second region of the dispersion of lesser electrical accessibility and adjacent said surface by connecting said second region with the first region of the dispersion having greater electrical accessibility to the counter-electrode through an ion conducting bridge which is impermeable to said ionized film-forming material but is permeable to said counter-ion and to ions having like sign to said counter-ion, said bridge communicating with said first and second regions and permitting the transfer of ions of like sign to said counter-ion from said second region to said first region under the influence of the electric potential gradient more readily than said ions would be transferred through said dispersion.

2. A process of coating according to claim 1 wherein the ion-conducting bridge is comprised of material selectively permeable to the counter-ion.

3. A process of coating according to claim 2 wherein the ion-conducting bridge is comprised of an ion exchanger.

4. A process of coating according to claim 1 wherein the ion-conducting bridge is comprised of material permeable to counter-ion and of a casing of higher mechanical strength.

5. A process of coating according to claim 1 wherein the ion-conducting bridge comprises an electrolyte or 7 polyelectrolyte enclosed within a casing which is permeable to counter-ion in the first and second regions of the dispersion.

6. A process of coating according to claim 5 wherein the casing comprises an ion exchanger.

7. A process of coating according to claim 5 wherein the counter-ion of the electrolyte or polyelectrolyte is the same as that of the dispersion.

8.-A process of coating according to claim 5 wherein the counter-ion of the electrolyte or polyelectrolyte is different from that of the dispersion.

9. A process of coating according to claim 1 wherein the ionised film-forming material is dispersed in an aqueous medium.

-10. A process of coating according to claim 9 wherein 15 the ionised film-forming material is a polycarboxylic acid resin at least partially neutralised with a base.

11. A process of coating according to claim 10 wherein the base counter-ion is derived from an amine and the counter-ion of an electrolyte or polyelectrolyte comprising the ion-conducting bridge is an inorganic base ion.

12. A process according to claim 11 wherein the counter-ion of an electrolyte or polyelectrolyte comprising the ion-conducting bridge is potassium.

References Cited UNITED STATES PATENTS HOWARD S. WILLIAMS, Primary Examiner 

